Oscillation prevention circuit

Abstract

This oscillation prevention circuit is characterized in that a charging switch circuit and a transfer switch circuit are provided in series between a first amplifier circuit and a second amplifier circuit; and a control circuit turning OFF the transfer switch circuit when the charging switch circuit is ON and also turning OFF the charging switch circuit when the transfer switch circuit is ON is provided to prevent the two switch circuits from being turned ON at the same time.

What is claimed is:  
     
         1 . An oscillation prevention circuit used in a microphone amplifier for one-way communication having a microphone, a first amplifier circuit, a second amplifier circuit, and a speaker or an earphone, wherein a charging switch circuit and a transfer switch circuit are provided in series between said first amplifier circuit and said second amplifier circuit; a transitional signal storage member is provided between the charging switch circuit and the transfer switch circuit; said charging switch circuit delivers a signal from the first amplifier circuit to the transitional signal storage member; said transfer switch circuit delivers the signal from the transitional signal storage member to the second amplifier circuit; and further a control circuit is provided which provide controls so that the transfer switch circuit is turned OFF when the charging switch circuit is ON and the charging switch circuit is turned OFF when the transfer switch circuit is ON to prevent both of the two switch circuits from being turned ON at the same time.  
     
     
         2 . The oscillation prevention circuit according to  claim 1 , wherein the microphone amplifier is partially or wholly inserted in use into an ear hole like a hearing aid.  
     
     
         3 . The oscillation prevention circuit according to  claim 1 , wherein the transitional signal storage member is a capacitor.  
     
     
         4 . The oscillation prevention circuit according to  claim 1 , wherein said control circuit comprises a first switch driver circuit for driving a charging switch circuit, a second switch driver circuit for driving the transfer switch circuit, a flip-flop circuit for alternately turning ON and OFF the first and second switch driving circuits to prevent these two switch circuits from being turned ON at the same time, and an oscillation circuit for making this flip-flop circuit operate.  
     
     
         5 . An oscillation prevention circuit used in a transmitter-receiver for interactive communication having a transmitter section comprising a microphone, a first amplifier circuit, a second amplifier circuit and a receiver section comprising a third amplifier circuit, a fourth amplifier circuit, and a speaker, wherein a charging switch circuit and a transfer switch circuit are provided in series between the first and second amplifier circuits, or between the third and fourth amplifier circuits; a transitional signal storage member is provided between the charging switch circuit and the transfer switch circuit; said charging switch circuit delivers a signal from the first amplifier circuit or the third amplifier circuit to the transitional signal storage member; said transfer switch circuit delivers a signal from the transitional signal storage member to the second amplifier circuit or the fourth amplifier circuit; and further a control circuit is provided which provide controls so that the transfer switch circuit is turned OFF when the charging switch circuit is ON and the charging switch circuit is turned OFF when the transfer switch circuit is ON to prevent both of the two switch circuits from being turned ON at the same time.  
     
     
         6 . The oscillation prevention circuit according to  claim 5 , wherein the transmitter-receiver is partially or wholly inserted in use into an ear hole like a hearing aid.  
     
     
         7 . The oscillation prevention circuit according to  claim 5 , wherein the transitional signal storage member is a capacitor.  
     
     
         8 . The oscillation prevention circuit according to  claim 5 , wherein the control circuit comprises a first switch driver circuit for driving the charging switch circuit, a second driver circuit for driving the transfer switch circuit, a flip-flop circuit for alternately turning ON and OFF the first and second switch driving circuits to prevent these two switch circuits from being turned ON at the same time, and an oscillation circuit for making this flip-flop circuit operate.  
     
     
         9 . The oscillation prevention circuit according to  claim 5 , wherein the transmitter-receiver is an earphone microphone having a hollow main body having an inserting section with an opening formed with a size adapted to an ear hole and enabling interactive communication with an external transmitter-receiver when the inserting section of said main body is dismountably set in an ear hole.  
     
     
         10 . The oscillation prevention circuit according to  claim 9 , wherein said earphone microphone has a microphone picking up audio signals propagating as air vibration from inside of an ear hole via an opening of the inserting section; and a speaker for amplifying audio signals received from an external transmitter-receiver in the main body, and further wherein this speaker is provided at a position opposite to the opening of the inserting section and is facing the opening; a central section of said microphone is provided at a position nearer to the opening of the inserting section than to the speaker and is substantially facing a center of said opening; and the microphone and the speaker are partitioned with a sound absorbing material so that mechanical vibration of the speaker is not directly delivered to the microphone.  
     
     
         11 . The oscillation prevention circuit according to  claim 10 , wherein the earphone microphone has a second microphone for picking up audio signals propagating through air from the outside provided on the external surface.  
     
     
         12 . The oscillation prevention circuit according to  claim 11 , wherein ON/OFF control for the second microphone and the microphone is provided so that the second microphone is turned ON and the microphone is turned OFF when the earphone microphone is not in the communication mode, and the second microphone is turned OFF and the microphone is turned ON when the earphone microphone is in the communication mode.